To detect the speed and direction of a rotating wheel or other object, it is common practice to attach a magnetically polarized ring to the wheel and position a magnetic field sensor nearby. As the alternating magnetic poles (north, south, north, south, etc.) pass by during rotation, the field sensor detects and converts the pole-sequence into a pulsed output voltage. The rotational speed of the wheel can then be derived by counting the pulses per second.
In order to obtain fine resolution, many poles on the ring are desired. Unfortunately, the magnetic field of the poles does not extend far beyond the ring itself; in fact, it decreases exponentially with the distance from the ring. The field typically disappears almost entirely at a distance that is about two to three pole-pitches away from the ring, where the pole-pitch is the distance between the centers of two adjacent poles on the ring.
Because of assembly tolerances, the distance between the field sensor and the pole-ring may vary. Thus, there is a need for a magnetic field sensor system that is compatible with both small (e.g., about 0.5 mm) and large distances (e.g., up to about several millimeters), in other words a magnetic field sensor that is sensitive to small magnetic fields.